In laser printers, a modulated light beam (or a plurality of beams) scans the surface of a moving photosensitive surface such as a rotating cylindrical photoreceptor to produce an image. The light beam typically scans the surface in the axial (scan) direction to produce each line of the image, while the cylinder rotates so that each line falls on a different cross-scan location on the surface. In some systems multiple lines are scanned simultaneously.
U.S. Pat. No. 5,268,687, to Peled et al, the disclosure of which is incorporated herein by reference, describes a printer in which errors in the rotational position of the cylinder or in the timing of the scan are compensated for by reflecting the light beam from a movable mirror, which adjusts the cross-scan position of the light beam. FIG. 1 of the present application shows FIG. 1 of the Peled et al. patent. The figure shows a cylinder 108, a source 110 for a light beam 111, and a movable mirror 112, mounted between a bearing 114 and a servo-motor 118 which rotates the mirror over a limited range on its axis. As can be seen from the drawing, the mirror is much longer than it is wide, and is of an appropriate length so that light beam 111 can reflect from it at any position in the axial scan of the light beam.
A feedback loop 129 uses the measured difference between the rotational position of the cylinder and an expected position at the beginning of a scan, as well as the measured orientation of the mirror, to drive servo-motor 118 which controls the orientation of the mirror, and hence the azimuthal location 116 where the light beam hits the surface of the cylinder. The feedback loop includes a controller 124 which receives signals about the rotational position of the cylinder from sensor 109, and signals about the timing of the scan from light source 110.
Hence controller 124 can determine the difference between the expected and actual rotational position of the cylinder at the beginning of each scan. Feedback loop 129 also includes a mirror control 120, which uses output from controller 124 to instruct servo-motor 118 how far and in what direction to rotate the mirror, and uses feedback from a sensor 122 which senses the orientation of the mirror.